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Celestial Equator

StarDate:

June 19, 2011

You can't see them, but all sorts of lines criss-cross the sky. They're like the lines that divide states and countries here on Earth, or that define sets of coordinates -- the equivalent of latitude and longitude.

One of those is the celestial equator -- the projection of Earth's equator on the sky. It's a little difficult to trace because there aren't many bright stars along its path. But tonight, the solar system helps point the way.

The planet Saturn is in the southwest at nightfall, and looks like a bright golden star. A true star is just a whisker away from it, so it looks like you're seeing double.

A little to the lower right of Saturn, the equator crosses another of those lines in the sky: the ecliptic, which traces the Sun's path across the sky. In fact, the point where the equator and ecliptic cross is where the Sun stands at the autumnal equinox in September.

By around 11:30 or 12, Saturn will have dropped pretty low in the west, and the Moon will just be climbing skyward in the east. The equator is to the left of the Moon by about the width of your fist held at arm's length.

The equator reaches its highest point about mid-way between Saturn and the Moon, in the middle of Ophiuchus, where it extends about halfway up the sky. Unfortunately, though, there's nothing there to mark it. So although it's one of the most important lines in the sky, the celestial equator just isn't much to look at.

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